1. Field of the Invention
This invention relates in general to a system for tensioning a string of casing extending between a subsea wellhead and a surface wellhead located on offshore platform, and in particular to a system utilizing an adjustable mandrel.
2. Description of the Prior Art
In certain types of offshore drilling, a string of casing will be connected between a subsea wellhead assembly at the sea floor and a surface wellhead at a platform located at the surface. For example, one technique involves drilling subsea wells with a floating drilling rig and leaving the wells cased but not completed for production. Later a production platform is installed over the subsea wellhead assemblies for completing the wells with surface wellheads at the platform. A tieback string of casing will be lowered from the platform and latched into the subsea assembly. The operator applies tension to the tieback string and adjusts a load shoulder at the surface wellhead for maintaining the tieback string in tension.
A number of different systems have been used and proposed in the past. Some of these systems employ a locking member which will ratchet on a mandrel in one direction and support weight in the other direction to maintain the string in tension. While these systems are workable, improvements to reduce cost and facilitate installation are desirable.